fix(object): clarify tool-call generation error

[ibetitsmike]

Problem

The error tool-based generation failed can be mistaken for a primary chat-generation failure. It is emitted by the structured-output object path when GenerateWithTool uses a forced tool call, including quick generation and title generation in coder/coder.

Change

Clarify the wrapper so it identifies both the operation and strategy:

• Before: tool-based generation failed
• After: structured output generation failed (tool-call strategy)

I checked the neighboring object strategy wrappers. There is no JSON-mode wrapper message in this file. The existing text and streaming strategy messages are unchanged because this PR is limited to the ambiguous non-streaming tool-call path from the incident.

Fork sync note

This error string is inherited from upstream, not fork-added. It was introduced by upstream commit cce6428 (feat: support structured output (#66)), exists at the fork's merge base with upstream/main, and remains present on the current upstream default branch. This PR intentionally creates a small fork divergence that future upstream syncs should preserve or reconcile.

In coder/coder, production code does not match this string. One quickgen test constructs the old wrapper text only to verify that errors.As still reaches the underlying *fantasy.ProviderError.

Tests

• go test ./object -count=1
• go test ./... -count=1 -timeout=30m
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run

CI status

The build test matrix passes on Linux, macOS, and Windows. The remaining repository-wide failures are unrelated to this one-line copy change and also occur on current PR #53: golangci-lint v2.9 cannot read Go 1.27 export data, and govulncheck reports vulnerabilities in the base branch's Go and dependency versions.

🤖 Mux authored this PR on Mike's behalf.